AOD vs AQI vs PM2.5 in India: A Simple Guide

When checking air pollution in India, you may encounter three common terms: AOD, AQI and PM2.5. Although all three relate to particles in the atmosphere, they do not measure the same thing.

In simple terms:

• AOD shows how much aerosol is present through a vertical column of the atmosphere.

• PM2.5 measures the concentration of fine particles near the ground.

• AQI converts multiple pollutant measurements into an easy-to-understand air-quality score.

Understanding these differences is essential when interpreting satellite maps, pollution-monitoring stations or air-quality apps.

Quick comparison: AOD vs AQI vs PM2.5

Feature	AOD	PM2.5	AQI
Full form	Aerosol Optical Depth	Particulate Matter ≤2.5 µm	Air Quality Index
What it represents	Aerosol loading through the atmospheric column	Fine-particle concentration near ground level	Overall air-quality condition
Unit	Dimensionless	µg/m³	Index from 0 to 500
Common source	Satellites and sun photometers	Ground monitoring stations and sensors	Calculated from pollutant measurements
Main use	Regional mapping and atmospheric research	Measuring human exposure to fine particles	Communicating health risk
Best for	Tracking smoke, dust and haze over large areas	Understanding fine-particle pollution	Deciding whether outdoor air is safe

What is AOD?

Aerosol Optical Depth, also called Aerosol Optical Thickness, measures how much sunlight is absorbed or scattered by aerosols while passing through the atmosphere.

Aerosols include:

• Dust

• Smoke

• Soot

• Sea salt

• Industrial particles

• Sulphates and nitrates

• Liquid droplets

AOD is generally measured using satellites such as MODIS and VIIRS or ground-based instruments such as sun photometers. It is a dimensionless value, meaning it is not measured in µg/m³.

A low AOD generally indicates clearer atmospheric conditions, while a high AOD suggests more aerosols, haze, smoke or dust. NASA describes AOD as an optical measurement of how suspended particles affect the transmission of light through the atmosphere. Learn more from NASA Earthdata.

However, AOD measures the entire vertical column of air between the surface and the satellite. It does not directly tell us how many particles are present at breathing level.

What is AOD useful for?

AOD is particularly useful for:

• Mapping pollution over regions without ground stations

• Tracking forest-fire and crop-residue smoke

• Monitoring dust storms

• Studying seasonal aerosol patterns

• Analysing pollution transport between states

• Estimating surface PM2.5 using statistical or machine-learning models

AOD provides excellent spatial coverage, but it should not be treated as a direct health-risk measurement.

What is PM2.5?

PM2.5 refers to airborne particles with an aerodynamic diameter of 2.5 micrometres or smaller. These particles are approximately 30 times smaller than the width of an average human hair.

PM2.5 may come from:

• Vehicle exhaust

• Coal-fired power plants

• Industrial emissions

• Construction and road dust

• Waste burning

• Crop-residue fires

• Diesel generators

• Household solid-fuel burning

• Chemical reactions involving gases in the atmosphere

Unlike AOD, PM2.5 is normally measured close to ground level and expressed in micrograms per cubic metre of air, written as µg/m³.

Because of their extremely small size, PM2.5 particles can penetrate deep into the lungs and may enter the bloodstream. Exposure is associated with respiratory and cardiovascular illnesses, including aggravated asthma, stroke, heart disease, chronic obstructive pulmonary disease and lung cancer. Children, older people, pregnant women and people with existing heart or lung conditions can be more vulnerable. World Health Organization: health effects of particulate pollution.

What is AQI?

The Air Quality Index is not a pollutant. It is a communication system that converts complex pollution measurements into a single number, colour and health category.

India’s National AQI covers eight pollutants:

1. PM2.5

2. PM10

3. Nitrogen dioxide (NO₂)

4. Sulphur dioxide (SO₂)

5. Carbon monoxide (CO)

6. Ground-level ozone (O₃)

7. Ammonia (NH₃)

8. Lead (Pb)

A separate sub-index is calculated for each available pollutant. The highest pollutant sub-index generally becomes the overall AQI.

This means that an AQI of 200 does not mean the air contains 200 µg/m³ of PM2.5. The AQI may also be driven by PM10, ozone or another pollutant.

The Central Pollution Control Board describes AQI as a tool that converts measurements of multiple pollutants into a single understandable index, category and colour. See the CPCB National AQI explanation.

Indian AQI categories and PM2.5 levels

For PM2.5, India uses the following 24-hour concentration breakpoints:

Indian AQI category	AQI range	PM2.5 concentration
Good	0–50	0–30 µg/m³
Satisfactory	51–100	31–60 µg/m³
Moderately Polluted	101–200	61–90 µg/m³
Poor	201–300	91–120 µg/m³
Very Poor	301–400	121–250 µg/m³
Severe	401–500	250+ µg/m³

These are India’s AQI breakpoints for converting PM2.5 concentration into a sub-index. They should not be confused with annual air-quality standards or WHO health guidelines.

Is AOD directly equal to PM2.5?

No. AOD and PM2.5 are related, but there is no universal formula that can accurately convert AOD directly into PM2.5 everywhere.

For example:

AOD 0.8 does not mean PM2.5 is 80 µg/m³, and it does not mean the AQI is 80.

The relationship depends on several factors:

• Height of the aerosol layer

• Atmospheric humidity

• Temperature

• Wind speed and direction

• Boundary-layer height

• Type and size of particles

• Surface reflectance

• Cloud cover

• Season and geographical location

Humidity can cause some particles to absorb water and scatter more light, increasing AOD without an equivalent increase in dry PM2.5 mass. Similarly, a dust or smoke layer high above the ground may produce high AOD while surface PM2.5 remains comparatively lower.

Reliable satellite-based PM2.5 estimation therefore requires a calibrated model combining AOD with meteorological information, land-use data and ground-monitoring observations.

Why can AOD be high while AQI is moderate?

This can happen when aerosols are located higher in the atmosphere rather than close to the ground.

Consider a dust storm passing over northern India. A satellite may detect a large amount of dust throughout the atmospheric column, producing high AOD. If most of that dust has not reached breathing level, the ground station may record only moderate pollution.

The reverse can also happen. Pollution trapped in a shallow layer near the ground during a winter temperature inversion can create very high PM2.5 and AQI, even when the total atmospheric-column aerosol load is not exceptionally high.

Why AOD, PM2.5 and AQI vary across India

India’s air pollution changes considerably with geography, weather and season.

Winter

Low temperatures, calm winds and a shallow boundary layer can trap emissions close to the surface. This often produces high PM2.5 and AQI across the Indo-Gangetic Plain.

Summer

Strong winds and dry conditions may generate mineral and road dust. AOD and PM10 can rise sharply, while PM2.5 may not increase by the same proportion.

Monsoon

Rain removes many particles from the atmosphere through wet deposition. AOD, PM2.5 and AQI therefore generally improve, although local pollution episodes can still occur.

Post-monsoon

Crop-residue burning, festival emissions, transport, industry and unfavourable weather can combine to produce widespread haze. Satellite AOD is useful for observing the regional extent of these pollution episodes.

Which value should you check?

The most useful measurement depends on your purpose.

For daily health decisions: Check AQI

AQI is the easiest value for deciding whether to exercise outdoors, send children for outdoor activities or take precautions during a pollution episode.

To understand particle exposure: Check PM2.5

PM2.5 provides the actual fine-particle concentration near the surface. Also check which pollutant is driving the AQI, because PM2.5 is not always the dominant pollutant.

For satellite mapping and regional analysis: Check AOD

AOD is useful for monitoring large pollution plumes, dust storms, smoke transport and areas where ground-monitoring stations are unavailable.

For the clearest assessment, use all three together:

AOD shows the regional atmospheric picture, PM2.5 measures fine particles near the ground, and AQI communicates the likely health concern.

How to respond when AQI is high

During poor, very poor or severe air-quality conditions:

• Reduce prolonged or strenuous outdoor activity.

• Check the dominant pollutant, not only the overall AQI.

• Keep children, older people and people with respiratory or heart conditions away from heavy outdoor exertion.

• Avoid outdoor exercise close to busy roads.

• Reduce indoor sources such as smoking, incense, waste burning and excessive frying.

• Use a properly fitted particulate-filtering mask when prolonged outdoor exposure cannot be avoided.

• Seek medical advice if pollution exposure causes persistent breathlessness, wheezing, chest pain or severe discomfort.

Frequently asked questions

Is PM2.5 the same as AQI?

No. PM2.5 is a pollutant concentration measured in µg/m³. AQI is a calculated index based on PM2.5 and other pollutants.

Is AOD the same as AQI?

No. AOD is a satellite or optical measurement of aerosol loading through the atmosphere. AQI is a ground-level public-health communication index.

Can satellites measure surface PM2.5?

Satellites usually observe aerosol-related optical properties rather than directly measuring surface PM2.5. Researchers estimate PM2.5 by combining satellite AOD with weather, atmospheric and ground-monitoring data.

Why do different air-quality apps show different AQI values?

Apps may use different monitoring stations, update times, averaging periods, interpolation methods or national AQI standards. Always confirm whether an app is displaying the Indian AQI, US AQI or its own calculated index.

Can AQI be high even when PM2.5 is not high?

Yes. AQI may be driven by PM10, ozone, nitrogen dioxide, carbon monoxide or another measured pollutant.

Conclusion

AOD, AQI and PM2.5 describe different aspects of air pollution.

• AOD measures how aerosols affect light through the atmospheric column.

• PM2.5 measures fine-particle concentration near breathing level.

• AQI converts multiple pollutant concentrations into a simple health-related index.

AOD is powerful for satellite-based mapping, PM2.5 is important for understanding fine-particle exposure, and AQI is the most practical indicator for everyday public communication. Using them together provides a much more complete picture of air pollution across India.